1. Field of the Invention
The present invention relates generally to emissive displays and, in particular, relates to a method and apparatus for automatically controlling the luminance or brightness of an emissive display, such as a vehicle instrument display, based on ambient lighting conditions.
2. Discussion of the Related Art
Historically, the brightness of an emissive vehicular instrument display was adjustable by an operator to compensate for the ambient lighting conditions. For example, an operator may wish to increase the brightness during the day, and decrease the brightness at night. However, adjusting the brightness during transitory conditions became an inconvenient and dangerous task, especially while operating a motor vehicle. Recently, automatic brightness control systems have operated by automatically adjusting the brightness of a vehicular instrument display as a function of input from an ambient light sensor. For instance, if an automobile traveled from a position of sunlight into a position of shade, an ambient light detector would sense the decrease in ambient illumination, and the brightness of the corresponding instrument displays would be decreased accordingly. Conversely, if the ambient light detector sensed an increase in ambient illumination, the brightness of the instrument displays would be increased.
Unfortunately, these automatic brightness control systems are incapable of properly compensating for relatively short time durations of a large illumination gradient of the ambient environment from one time to the next. Accordingly, if a vehicle were to travel under a bridge for a short period of time, for example, the brightness of the instrument displays would be decreased, and then immediately increased once the vehicle traveled past the bridge. For such transitory conditions, it is annoying and unacceptable to the operator of the vehicle to change the display brightness at or faster than the adaptation rate of the eye. Rather, it is more desirable to have the display remain at a constant brightness level unless the ambient lighting condition is more permanent in nature, such as is the case when a vehicle passes under an elongated tunnel.
What is therefore needed is an automatic brightness control system that (1) allows an operator to compensate for his or her personal preference by manually adjusting the brightness of the display; and (2) automatically adjusts the rate of brightness change based on the magnitude of deviation of the current display brightness from the desired display brightness.
A method and apparatus are provided having the ability to adjust the rate of brightness change of a display based on the magnitude of deviation of the current display brightness from the desired illumination, wherein the desired brightness is a function of the ambient illumination. Furthermore, the automatic brightness control system allows an operator to manually alter the determined brightness if desired.
In accordance with a first aspect of the invention, the brightness control system includes a filter that waits a predetermined amount of time according to the difference between the current display brightness and the desired display brightness. For example, if the difference between the two brightness levels is large, the process will wait a relatively long period of time before transitioning the display brightness to the desired display brightness when compared to the situation where the difference between the two values is relatively small. As a result, when an automobile travels underneath a bridge, for example, the magnitude of the difference will be relatively large for a relatively short period of time. The display brightness would likely not change in this scenario because the necessary time would not have elapsed before the ambient sensor detected a subsequent increase in illumination (as the vehicle exits from under the bridge). Specifically, the filter uses a predetermined time constant that is multiplied by the difference between the current and the desired brightness to determine the necessary time elapse before the current display brightness is adjusted.
In accordance with another aspect of the invention, an additional predetermined time interval is used to determine whether sufficient time has passed to update the DAC count which will, in turn, update the display brightness.
In accordance with another aspect of the invention, the display brightness increases at a faster rate than it decreases. Specifically, a smaller time constant is used when brighter ambient conditions are sensed, thereby resulting in a smaller update time interval. Once the change in time from the previous update has exceeded the update time, the brightness step number will be incremented or decremented, and the process will revert to the beginning of the update cycle. Therefore, the brightness control system may be used as a brightness peak detector, because the level of maximum brightness will not decrease as quickly as the brightness level is increased.
These as well as other features, aspects, and characteristics of the present invention will be apparent from the description which follows. In the detailed description below, preferred embodiments of the invention will be described with reference to the accompanying drawings. These embodiments do not represent the full scope of the invention. Rather, the invention may be employed in other embodiments. Reference should therefore be made to the claims herein for interpreting the breadth of the invention.